Karl E. Haisch

Disk Frequencies and Lifetimes in Young Clusters

We report the results of the first sensitive L-band survey of the intermediate-age (2.5-30 Myr) clusters NGC 2264, NGC 2362, and NGC 1960. We use JHKL colors to obtain a census of the circumstellar disk fractions in each cluster. We find disk fractions of 52% ± 10%, 12% ± 4%, and 3% ± 3% for the three clusters, respectively. Together with our previously published JHKL investigations of the younger NGC 2024, Trapezium, and IC 348 clusters, we have completed the first systematic and homogeneous survey for circumstellar disks in a sample of young clusters that both span a significant range in age (0.3-30 Myr) and contain statistically significant numbers of stars whose masses span nearly the entire stellar mass spectrum. Analysis of the combined survey indicates that the cluster disk fraction is initially very high (≥80%) and rapidly decreases with increasing cluster age, such that one-half the stars within the clusters lose their disks in 3 Myr. Moreover, these observations yield an overall disk lifetime of ~6 Myr in the surveyed cluster sample. This is the timescale for essentially all the stars in a cluster to lose their disks. This should set a meaningful constraint for the planet-building timescale in stellar clusters. The implications of these results for current theories of planet formation are briefly discussed.

Infrared L-Band Observations of the Trapezium Cluster: A Census of Circumstellar Disks and Candidate Protostars

We report the results of a sensitive near-infrared JHKL imaging survey of the Trapezium cluster in Orion. We use the JHKL colors to obtain a census of infrared excess stars in the cluster. Of (391) stars brighter than 12th magnitude in the K and L bands, 80% ± 7% are found to exhibit detectable infrared excess on the J-H, K-L color-color diagram. Examination of a subsample of 285 of these stars with published spectral types yields a slightly higher infrared excess fraction of 85%. We find that 97% of the optical proplyds in the cluster exhibit excess in the JHKL color-color diagram indicating that the most likely origin of the observed infrared excesses is from circumstellar disks. We interpret these results to indicate that the fraction of stars in the cluster with circumstellar disks is between 80%–85%, confirming earlier published suggestions of a high disk fraction for this young cluster. Moreover, we find that the probability of finding an infrared excess around a star is independent of stellar mass over essentially the entire range of the stellar mass function down to the hydrogen burning limit. Consequently, the vast majority of stars in the Trapezium cluster appear to have been born with circumstellar disks and the potential to subsequently form planetary systems, despite formation within the environment of a rich and dense stellar cluster. We identify 78 stars in our sample characterized by K-L colors suggestive of deeply embedded objects. The spatial distribution of these objects differs from that of the rest of the cluster members and is similar to that of the dense molecular cloud ridge behind the cluster. About half of these objects are detected in the short wavelength (J and H) bands, and these are found to be characterized by extreme infrared excess. This suggests that many of these sources could be protostellar in nature. If even a modest fraction (i.e., ~50%) of these objects are protostars, then star formation could be continuing in the molecular ridge at a rate comparable to that which produced the foreground Trapezium cluster.

Circumstellar Disks in the IC 348 Cluster

We report the results of the first sensitive L-band (3.4 μm) imaging survey of the young IC 348 cluster in Perseus. In conjunction with previously acquired JHK (1.25, 1.65, and 2.2 μm) observations, we use L-band data to obtain a census of the circumstellar disk population to mK = mL ≤ 12.0 in the central ~110 arcmin2 region of the cluster. An analysis of the JHKL colors of 107 sources indicates that 65% ± 8% of the cluster membership possesses (inner) circumstellar disks. This fraction is lower than those (86% ± 8% and 80% ± 7%) obtained from similar JHKL surveys of the younger NGC 2024 and Trapezium clusters, suggesting that the disk fraction in clusters decreases with cluster age. Sources with circumstellar disks in IC 348 have a median age of 0.9 Myr, while the diskless sources have a median age of 1.4 Myr, for a cluster distance of 320 pc. Although the difference in the median ages between the two populations is only marginally significant, our results suggest that over a timescale of ~2–3 Myr more than a third of the disks in the IC 348 cluster disappear. Moreover, we find that at a very high confidence level the disk fraction is a function of spectral type. All stars earlier than G appear diskless, while stars with spectral types G and later have a disk fraction ranging between 50%–67%, with the latest-type stars having the higher disk fraction. This suggests that the disks around stars with spectral types G and earlier have evolved more rapidly than those with later spectral types. The L-band disk fraction for sources with similar ages in both IC 348 and Taurus is the same within the errors, suggesting that at least in clusters with no O stars the disk lifetime is independent of environment.

A Near-Infrared L-Band Survey of the Young Embedded Cluster NGC 2024

We present the results of the first sensitive L-band (3.4 μm) imaging study of the nearby young embedded cluster NGC 2024. Two separate surveys of the cluster were acquired in order to obtain a census of the circumstellar disk fraction in the cluster. We detected 257 sources to the mL ≤ 12.0 completeness limit of our ~110 arcmin2 primary survey region. An additional 26 sources with 12.0 < L < 14.0 were detected in the deeper survey of the central ~6.25 arcmin2 region of the cluster. From an analysis of the JHKL colors of all sources in our largest area, we find an infrared excess fraction of ≥86% ± 8%. The JHKL colors suggest that the infrared excesses arise in circumstellar disks, indicating that the majority of the sources that formed in the NGC 2024 cluster are currently surrounded by, and likely formed with, circumstellar disks. The excess fractions remain very high, within the errors, even at the faintest L magnitudes from our deeper surveys, suggesting that disks form around the majority of the stars in very young clusters such as NGC 2024 independent of mass. From comparison with published JHKL observations of Taurus, we find the K-L excess fraction in NGC 2024 to be formally higher than in Taurus, although both fractions are quite high. Thus, existing L-band observations are consistent with a high initial incidence of circumstellar disks in both NGC 2024 and Taurus. Because NGC 2024 represents a region of much higher stellar density than Taurus, this suggests that disks may form around most of the YSOs in star-forming regions independent of environment. We find a relatively constant JHKL excess fraction with increasing cluster radius, indicating that the disk fraction is independent of location in the cluster. In contrast, the JHK excess fraction increases rapidly toward the central region of the cluster. The most likely cause for this increase is the contamination of the K-band measurements by bright nebulosity in the central regions of the cluster. This suggests that caution must be applied using only JHK-band observations to infer disk fractions in nebulous environments. Finally, we identify 45 candidate protostellar sources in the central regions of the cluster, and we find a lower limit on the protostellar phase of early stellar evolution in the NGC 2024 cluster of 0.4–1.4 × 105 yr, similar to that in Taurus.

A Near-Infrared Multiplicity Survey of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We present new near-IR observations of 76 Class I/flat-spectrum objects in the nearby (d 320 pc) Perseus, Taurus, Chamaeleon I and II, ρ Ophiuchi, and Serpens dark clouds. These observations are part of a larger systematic infrared multiplicity survey of self-embedded objects in the nearest dark clouds. When combined with the results of our previously published near-infrared multiplicity survey, we find a restricted companion star fraction of 14/79 (18% ± 4%) of the sources surveyed to be binary or higher order multiple systems over a separation range of ~300–2000 AU with a magnitude difference ΔK ≤ 4 and with no correction for background contamination or completeness. This is consistent with the fraction of binary/multiple systems found among older pre–main-sequence T Tauri stars in each of the Taurus, ρ Oph, and Chamaeleon star-forming regions over a similar separation range, as well as the combined companion star fraction for these regions. However, the companion star fraction for solar-type, and lower mass M dwarf, main-sequence stars in the solar neighborhood in this separation range (11% ± 3%) is approximately one-half that of our sample. Together with multiplicity statistics derived for previously published samples of Class 0 and Class I sources, our study suggests that a significant number of binary/multiple objects may remain to be discovered at smaller separations among our Class I/flat-spectrum sample and/or most of the evolution of binary/multiple systems occurs during the Class 0 phase of early stellar evolution.

A Mid-Infrared Study of the Young Stellar Population in the NGC 2024 Cluster

We present the results of the first broadband N (10.8 μm) survey of the NGC 2024 cluster. The mid-infrared data were combined with our previously published JHKL photometry to construct spectral energy distributions for all detected sources. The main scientific goals were to investigate the nature of the young stellar objects (YSOs) in the cluster and to examine the efficiency of detecting circumstellar disk sources from near-infrared JHKL color-color diagrams. Out of 59 sources surveyed having K-band (2.2 μm) magnitudes mK ≤ 10.5, we detected 35 (~59%) at 10 μm. Combining these detections and upper limits for the nondetections with existing JHKL data, we identify one Class I, six flat-spectrum, 28 Class II, and five Class III sources. We find a circumstellar disk fraction for NGC 2024 of ~85% ± 15%, which confirms earlier published suggestions that the majority, if not all, of the stars in the NGC 2024 cluster formed with disks, and these disks still exist at the present time. In addition, all but one of the disk sources identified in our survey lie in the infrared-excess region of the JHKL color-color diagram for the NGC 2024 cluster. This demonstrates that JHKL color-color diagrams are extremely efficient in identifying YSOs with circumstellar disks. Of the 14 sources in our survey with K-L colors suggestive of protostellar objects, ~29% are protostellar in nature, while ~7% are true Class I sources. This may be due to extinction producing very red K-L colors in Class II YSOs, thus making them appear similar in color to protostars. This suggests that caution must be applied when estimating the sizes and lifetimes of protostellar populations within star-forming regions based on K-L colors alone. A comparison of the ratio of the number of Class I and flat-spectrum sources to the number of Class II and III sources in NGC 2024, ρ Oph, and Taurus-Auriga indicates that NGC 2024 and ρ Oph have similar ages, while Taurus-Auriga is an older region of star formation, consistent with published T Tauri star ages in each region. Finally, we calculate the luminosities of the Class II sources in NGC 2024, ρ Oph, and Taurus and discuss the results.

FIRST EVIDENCE OF A PRECESSING JET EXCAVATING A PROTOSTELLAR ENVELOPE

We present new, sensitive, near-infrared images of the Class I protostar, Elias 29, in the Ophiuchus cloud core. To explore the relationship between the infall envelope and the outflow, narrowband H_2 1-0 S(1), Brγ, and K_(cont) filters were used to image the source with the Wide-Field Infrared Camera on the Hale 5 m telescope and with Perssons Auxiliary Nasmyth Infrared Camera on the Baade 6.5 m telescope. The source appears as a bipolar, scattered light nebula, with a wide opening angle in all filters, as is typical for late-stage protostars. However, the pure H_2 emission-line images point to the presence of a heretofore undetected precessing jet. It is argued that high-velocity, narrow, precessing jets provide the mechanism for creating the observed wide-angled outflow cavity in this source.